The invention relates to a method of manufacturing a window transparent to electron rays as well as to a window transparent to electron rays comprising a foil which is transparent to electron rays and an element for supporting a peripheral region of the foil transparent to electron rays in the operational state, which element is made from a material having a greater linear thermal expansion coefficient than the foil material. The invention further relates to an X-ray device fitted with such a window.
Such windows transparent to electron rays are used wherever sensitive objects are to be screened off from external conditions while still a sufficient transparency for the passage of the electron ray is to be safeguarded. The use of such windows in an X-ray tube with a metal target, also denoted LIMAX X-ray tube (LIMAX=Liquid Metal Anode X-ray tube), is proposed in DE 198 21 939 A1. Such an X-ray device basically consists of an electron source and a target of liquid metal which circulates in the operational condition of the radiation device. The liquid metal is present in a pump circulation system and is pumped into a receiver container via a special steel plate by means of a distribution head. The electron ray hits the liquid metal flowing over the special steel plate and generates X-radiation therein. It is achieved by means of the window that the vacuum space of the electron source and the target are separated from one another into two independent spaces, so that the target becomes less sensitive to the flow characteristics and the type of liquid metal chosen. Such a window comprises, for example, a 1 xcexcm thick diamond layer vapor-deposited on a silicon carrier substrate in a CVD process, from which then the carrier substrate is partly or wholly removed for creating a window or transmission zone. The window thus constructed is directly mounted in the X-ray tube.
It was found that windows manufactured in this way are not resistant to pressure differences of more than 4 bar such as they occur, for example, in a LIMAX X-ray tube in accordance with DE 198 21 939 A1, because at higher pressure differences the diamond film is torn from the carrier substrate, which at the same time serves as a retaining element, so that the window is destroyed. The bursting pressure is reached in particular during the starting phase of tube operation in the case of LIMAX tubes, when pressure differences of more than 4 bar occur.
The invention has for its object to provide a method of manufacturing a window transparent to electron rays as well as a window transparent to electron rays wherein the disadvantages mentioned above do not arise.
This object is achieved by means of a method of the kind mentioned in the opening paragraph in that a foil transparent to electron rays is manufactured, preferably having a thickness smaller than 10 xcexcm and more preferably smaller than 5 xcexcm, and in that said foil is connected to the retaining element via an intermediate layer consisting of a material having a linear thermal expansion coefficient equal or similar to the linear thermal expansion coefficient of the material of the foil and smaller than the linear thermal expansion coefficient of the material of the retaining element, seen over the processing temperature range, and in that the intermediate layer forms a buffer for the difference in thermal expansion characteristics between the retaining element and the foil during the joining process.
A window with a foil of high mechanical robustness is achieved by this joining technique, said foil being fixedly connected to the retaining element, which positively influences the operational life of the window as well as its loading limits.
In principle, the proposed method is suitable also for joining the foil to the retaining element at room temperature, but it only provides its specific advantages in joining processes under heat supply, for example soldering with soldering or joining temperatures of  greater than 100xc2x0 C., usually 400-900xc2x0 C. Joining of the foil to the retaining element by way of the interposed protective intermediate layer, i.e. via a stress buffer, avoids that the foil upon cooling down is deformed or folded owing to the shrinking action of the retaining element caused by its greater linear thermal expansion coefficient. The planar thin foil or window surface which is not or substantially not deformed leads to a reduction or total elimination of hydrodynamic dead water zones when the window according to the invention is introduced into flowing media, which is of particular benefit for the cooling properties of the window for fluids to be cooled which flow past the window surface. The window thus manufactured and constructed is suitable for use as a separation means in overpressure or underpressure chambers, in particular for pressure differences of more than 0.1 bar, more in particular above 5 bar, or alternatively as a separation means for chambers having different contents such as gases or fluids of different compositions.
The manufacture according to the invention may take place in a single-step process or a two-step process. In the former embodiment of the method, the foil transparent to electron rays, the intermediate layer, and the retaining element are joined together in a single collective step. In the latter embodiment of the method, the foil transparent to electron rays is connected to the intermediate layer in a first step so as to obtain a layer package, and in a subsequent second step this layer package is joined to the retaining element.
The connection itself is achieved by means of suitable adhesion or fusion layers. Eligible fusion processes are all known processes, in particular fusion by means of an active metal solder or glass fusion. Adhesive bonding layers, in particular heat-activated glue layers, or alternatively combined glue-fusion layers are also possible. Preferably, ceramic glues (for example, marketed by the Aremco Company) should also be used. The processing temperatures are then to be chosen in dependence on the adhesion and fusion substances used, lying, for example, between room temperature and the fusion temperature of, for example, the active solder.
In a preferred embodiment, it is suggested that the thin foil transparent to electron rays is connected to an intermediate layer whose thickness is equal to, or preferably greater than the thickness of the foil so as to obtain a layer package of greater rigidity, which layer package is connected to the retaining element via a connecting layer (adhesion or fusion layer).
The thin foil is stabilized over the intermediate layer in a first step, and the connection to the retaining element, whose coefficient of thermal expansion is greater than that of the material of the foil transparent to electron rays, then takes place by way of this intermediate layer or layer package. Said thicker intermediate layer or layer package absorbs a major portion of the stresses arising from the differences in expansion coefficient on account of its greater rigidity and thus protects the thin foil transparent to electron rays.
In a further embodiment it is proposed that the foil transparent to electron rays is connected to a first partial intermediate layer and subsequently to at least a second partial intermediate layer. It is alternatively possible that first the intermediate layer comprising at least two partial intermediate layers is manufactured, which is then joined to the foil transparent to electron rays. Generally, a layer stack is manufactured first each time and is subsequently joined to the retaining element.
It should be noted here that a vacuum separation window transmitting light rays, i.e. a window of a different kind, is known from DE 43 01 146 A1, wherein the light comprises an X-ray, infrared, visible, and ultraviolet radiation. The vacuum window consists of a thin layer transmitting the radiation with a carrier element which supports this thin layer. Between the thin layer and the carrier element there is a layer by means of which the stresses acting on the thin layer are reduced, i.e. stresses which have arisen owing to differences in coefficients of thermal expansion of the materials of the carrier element and the thin layer not during the manufacturing process, but during the heat treatment for achieving an ultra-high vacuum (thermal drying). This intermediate layer is to be composed of a metal or an alloy which produces a liquid in that temperature range which corresponds to the operating temperature. In particular, gallium, a gallium-indium alloy, and a gallium-tin alloy are suggested as the liquid metal or liquid alloy for this intermediate layer, having a sufficient viscosity and surface tension and in addition a low vapor pressure. In contrast thereto, the intermediate layer proposed by the present invention is solid in the operational state.
According to the invention, the peripheral region, i.e. the edges, of the foil transparent to electron rays is connected to the retaining element via the intermediate layer, the latter as well as the former being provided with an opening, i.e. being, for example, annular in shape. It is not absolutely necessary here that the edges of the intermediate layer or partial intermediate layers adjoining the transmission zone for the electron ray are formed perpendicular to the longitudinal axis of the foil, they may also have sloping or curved edge shapes. It is also conceivable in principle that the intermediate layer is transparent to electron rays together with the foil and extends over the surface as does the foil.
According to the invention, a window transparent to electron rays of the type indicated is further formed with an intermediate layer which consists of a material having a linear thermal expansion coefficient which is equal or similar to, preferably greater than, the linear thermal expansion coefficient of the foil material and smaller than the linear thermal expansion coefficient of the material of the retaining element, seen over the relevant processing temperature range.
Preferably, the foil transparent to electron rays is made of diamond with a thickness of no more than 10 xcexcm. In an advantageous embodiment, the foil may alternatively be made of molybdenum, which may be as thin as 3 xcexcm, or of beryllium.
In the case of a diamond foil, the material of the intermediate layer has a linear thermal expansion coefficient smaller than 5xc3x9710xe2x88x926/K; and preferably the linear thermal expansion coefficient should be at most four times the linear thermal expansion coefficient of diamond, which lies at approximately 0.5 to 1xc3x9710xe2x88x926/K. The linear thermal expansion coefficient of ideal diamond as a monocrystal lies at 0.5xc3x9710xe2x88x926/K, which coefficient rises to a value of up to 1xc3x9710xe2x88x926/K in the manufacture by a CVD process and the accompanying formation of polycrystalline material.
Suitable materials to be used for the intermediate layer in combination with matching adhesion or fusion agents are, besides diamond, particularly quartz glass, silicon, Si3N4, SiO2, SiC, as well as industrial tool ceramics of low thermal expansion coefficient between 1.5 and 2xc3x9710xe2x88x926/K such as, for example, SiAlON. Included are also materials whose linear thermal expansion coefficient is smaller than that of in particular technically manufactured diamond. Preferred materials for the retaining element are given in claim 15. All possible combinations of materials for the foil, the intermediate layer, and the retaining element are conceivable and form part of the invention.
The intermediate layer provided in accordance with the invention should have a thickness equal to or greater than the foil thickness. The thickness preferably lies in a region between the values of 5 and 5000 xcexcm.
To buffer the heat-induced stresses better in the bridging zone formed by the intermediate layer, it is advantageous to subdivide the intermediate layer into partial intermediate layers and to provide at least one of the partial intermediate layers with a cooling element. Such a cooling element may be, for example, a cooling channel which is provided in the layer by means of a laser. Cooling liquids which may be used are water, oil, liquid metals, etc.. If the window transparent to electron rays is used in a LIMAX X-ray device, the cooling channel may advantageously be incorporated into the cooling circulation system thereof.
It is advisable in the case of a diamond foil to reduce the electrical resistance of the diamond, in particular of the diamond foil transparent to radiation, through doping, for example with boron, so as to reduce or avoid a deflection of the electron ray.
Furthermore, the thickness of the diamond foil transparent to electron rays should comply with:
window thickness (xcexcm) greater than 0.7 L (cm)xc3x97xcex94p (bar), with xcex94p (bar) being the pressure difference between the two window sides and L being the greatest longitudinal dimension of the window opening, i.e. L being the diameter in the case of circular openings, the major axis in the case of elliptical openings, and the longest side in the case of rectangular openings.